Match the words in the left hand column to the appropriate blank in the sentences in the right hand column. Use each word only once.

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1 Chapter 15 Homework Due: 11:59pm on Sunday, November 13, 2016 To understand how points are awarded, read the Grading Policy for this assignment. Question 1 Match the words in the left hand column to the appropriate blank in the sentences in the right hand column. Use each word only once. Hint 1. What are spiral galaxies? Spiral galaxies look like flat, white disks with yellowish bulges at their centers. The disks are filled with cool gas and dust, interspersed with hotter ionized gas, and usually display beautiful spiral arms. Hint 2. What are spiral arms? Spiral arms are bright, prominent arms, usually in a spiral pattern, found in most spiral galaxies. Hint 3. What is the disk? A portion of a spiral galaxy looks like a disk and contains an interstellar medium with cool gas and dust; stars of many ages are found in the disk component. Hint 4. What is the bulge? The central portion of a spiral galaxy is roughly spherical (or football shaped) and bulges above and below the plane of the galactic disk to form the bulge. Hint 5. What is the halo? The spherical region surrounding the disk of a spiral galaxy is the halo. Hint 6. What are globular clusters? Spherical clusters of a million or more stars, globular clusters are found primarily in the halos of galaxies and contain only very old stars. Reset Help 1. The circular but relatively flat portion of the galaxy is the disk. 2. The first portion of the galaxy to form was the halo. 3. A tightly packed group of a few hundred thousand very old stars is a globular cluster. 4. A spiral arm stands out in a photo of a galaxy because it shines brightly with light from massive young stars and glowing clouds of gas and dust. 5. Our Milky Way galaxy is a spiral galaxy. 6. Stars orbiting in the bulge near the galaxy's center can have orbits highly inclined to the galactic plane. 1/8

2 Question 2 Which part of the galaxy contains the coldest gas? the disk the halo the bulge Question 3 Why do disk stars bob up and down as they orbit the galaxy? because the gravity of other disk stars always pulls them toward the disk because of friction with the interstellar medium because the halo stars keep knocking them back into the disk Question 4 Which part of the galaxy has gas with the hottest average temperature? the disk the halo the bulge Question 5 The Sun's location in the Milky Way Galaxy is. 2/8

3 in the halo of the galaxy, about 28,000 light years above the galactic disk at the very outer edge of the galactic disk in the galactic disk, roughly halfway between the center and the outer edge of the disk very near the galactic center Question 6 How do disk stars orbit the center of the galaxy? They have orbits randomly inclined and in different directions relative to the galactic center. They follow spiral paths along the spiral arms. They all orbit in roughly the same plane and in the same direction. They follow orbits that move up and down through the disk, typically taking them about 50,000 light years above and below the disk on each orbit. Question 7 How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path? by estimating the amount of gas and dust in between the stars by using the law of conservation of angular momentum to calculate the orbital speeds of nearby stars by applying Newton's version of Kepler's third law (or the equivalent orbital velocity law) to the Sun's orbit around the center of the galaxy by counting the number of stars visible in this region of the galaxy Question 8 What do we mean by the star gas star cycle? It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium. It describes the orbits of the stars and interstellar medium around the center of the galaxy. It is the set of nuclear reactions by which heavy elements are produced in the cores of massive stars. It is the idea that stars in close binary systems can exchange gas with one another. 3/8

4 Question 9 Where does most star formation occur in the Milky Way Galaxy? everywhere throughout the galactic disk in the central bulge in the spiral arms within the halo Question 10 Each item below belongs either with the population of disk stars or the population of halo stars of the Milky Way Galaxy. Match each item to the appropriate population. Hint 1. How do the disk and halo differ in shape? The basic difference between the disk and halo in shape is. the disk is flat and the halo is spherical the halo is flat and the disk is spherical the disk rotates in one direction and the halo rotates precisely opposite the disk Hint 2. Where do we find star forming clouds? Clouds of gas and dust that are currently giving birth to stars are found in. only the disk only the halo both the disk and halo Hint 3. Which part of our galaxy formed first? As gravity shrunk the protogalactic cloud that gave birth to our galaxy, the first region to take shape was. the disk the halo 4/8

5 Question 11 The best measurements of the mass of the black hole at the galactic center come from the orbits of stars in the galactic center. the amount of radiation coming from the galactic center. the orbits of gas clouds in the galactic center. Question 12 How do we determine the Milky Way's mass outside the Sun's orbit? from the Sun's orbital velocity and its distance from the center of our galaxy from the orbits of halo stars near the Sun from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun 5/8

6 Question 13 Based on observations, which of the following statements about stars in the Milky Way is generally true? The older the star, the faster its orbital speed. The less massive the star, the older it is. The older the star, the lower its abundance of heavy elements. The older the star, the bluer its color. The younger the star, the higher its mass. Question 14 Listed following are several locations in the Milky Way Galaxy. Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest. Hint 1. How wide is the disk of the Milky Way Galaxy? The diameter of the disk of the Milky Way Galaxy is approximately. 10,000 light years 28,000 light years 100,000 light years Hint 2. How far from the center of the Milky Way Galaxy is our solar system? The distance between the center of the Milky Way Galaxy and our solar system is approximately. 10,000 light years 28,000 light years 100,000 light years Hint 3. How wide is the central bulge of the Milky Way Galaxy? The distance across the central bulge of the galaxy is approximately. 10,000 light years 28,000 light years 100,000 light years Hint 4. How far does the halo of the Milky Way Galaxy extend? Suppose you observe a globular cluster located about 70,000 light years from the center of our galaxy. This cluster is. 6/8

7 within the central bulge of our galaxy far out in the halo of our galaxy so far away that it must be in another galaxy Reset Help Farthest Closest a globular cluster in the outskirts of the halo a cloud of gas and dust in the outskirts of the disk our solar system the edge of the central bulge The central bulge extends out about 5,000 light years from the galactic center. Our solar system is about 28,000 light years from the center, which is about halfway through the disk. The halo extends to much greater distances than the 50,000 light year diameter of the disk, which is why this choice is ranked first. Part B Imagine a photon of light traveling the different paths in the Milky Way described in the following list. Rank the paths based on how much time the photon takes to complete each journey, from longest to shortest. Hint 1. How long does it take light to travel one of the paths? The time it takes a photon of light to travel one of the paths given in Part B depends on. whether the path is in the disk or the halo the distance along the path both the distance along the path and whether the path is in the disk or the halo Hint 2. How thick is the galactic disk? Relative to its diameter, the thickness of the galactic disk is. 7/8

8 about as thick as a car tire relative to the tire s diameter about 10 percent as thick as the disk s diameter flatter than a pancake Reset Help Longest time Shortest time across the diameter of the galactic halo across the diameter of the galactic disk from the Sun to the center of the galaxy across the diameter of the central bulge through the disk from top to bottom The light travel time converts directly to a distance in light years, so you can think of this question as a simple variation on, but with the added choice of the thickness of the disk (from top to bottom). The correct answer shows that the halo is larger in diameter than the disk, which is larger than the bulge, and that the Sun is within the disk but outside the bulge. The disk is quite thin compared to its diameter (typically only about 1,000 light years in thickness), which makes it relatively flatter than a typical pancake. Question 15 What kind of object do we think lies in the center of the Milky Way Galaxy? a gigantic X ray binary system a 3 to 4 million solar mass black hole an enormous collection of dark matter, explaining why we detect no light at all from the galactic center a dense cluster of young, hot stars Score Summary: Your score on this assignment is 101%. You received out of a possible total of 39 points. 8/8